1. General corrosion characteristics of brass
Brass is a Cu-Zn alloy with Zn as the main alloy element. It is called brass because of its yellow color. According to the types and contents of added alloying elements, brass can be divided into three categories: single-phase brass, complex-phase brass and special brass. When the zinc content is less than 36%, a single-phase α solid solution is formed, so single-phase brass is also called α brass. When the zinc content is 36%-45%, it becomes α+β complex phase brass. When the zinc content is greater than 45%, there is too much β phase and it is brittle and has no practical value. Special brass is based on Cu-Zn, and adds Sn, Mn, Al, Fe, Ni, Si, Pb and other elements.
Brass corrodes very slowly in the atmosphere, the corrosion rate in pure fresh water is not large (0.0025-0.025mm/year), and the corrosion rate in seawater is slightly faster (0.0075-0.1mm/year). Fluoride in the water has little effect on the corrosion of brass, chloride has a greater effect, and iodide has a serious effect. In water containing gases such as O2, CO2, H2S, SO2, NH3, etc., the corrosion rate of brass increases dramatically. It is extremely easy to corrode in mineral water, especially water containing Fe2(SO4)3. It causes serious corrosion in nitric acid and hydrochloric acid, corrodes slowly in sulfuric acid, and is resistant to corrosion in NaOH solution. Brass has better impact corrosion resistance than pure copper.
Special brass has better corrosion resistance than ordinary brass. Adding about 1% Sn to brass can significantly reduce the dezincification corrosion of brass and improve the corrosion resistance in seawater; adding about 2% Pb to brass can increase the wear resistance, thus greatly Reduces its corrosion rate in flowing seawater. In order to prevent dezincification corrosion, a small amount of As, Sb, and P (0.02%~0.05%) can also be added; navy brass contains 0.5%~1.0% Mn, which can improve strength and have good corrosion resistance. . In brass containing 65% Cu and 55% Cu, 12%-18% Ni is used to replace part of Zn. Because the color is silvery white, it is called nickel silver or German silver. This alloy has excellent corrosion resistance in salts, alkalis and non-oxidizing acids. At the same time, since a large amount of Ni replaces 2n, there is no dezincification phenomenon. In addition to the above corrosion characteristics, brass also has two important forms of corrosion, namely dezincification corrosion and stress corrosion.
2. Stress corrosion cracking of brass
The factors that affect the stress corrosion cracking of brass include corrosive media, stress, alloy composition and organizational structure. A certain alloy will corrode and crack only under certain media and specific stress conditions.
(1) Corrosive medium
Brass under tensile stress can produce stress corrosion in all ammonia-containing (or NH4+) media and in the atmosphere, sea water, fresh water, high temperature and high pressure water, and water vapor. For example, cracking of brass bullet casings during the summer rainy season (also known as seasonal cracking) is a typical example of brass stress corrosion cracking. In addition, the stress corrosion cracking morphology of brass is divided into intergranular and transgranular. In the film-forming solution, intergranular fracture mainly occurs, and in the non-film-forming solution, transgranular fracture mainly occurs. The stress corrosion cracking mechanism of brass is generally believed to be that in the film-forming solution, a layer of cuprous oxide film with poor toughness is formed on the surface of brass. Under the action of stress and strain, the cuprous oxide film undergoes brittle cracking, and then forms at the grain boundaries. After this film is brittle, the crack will extend to the base metal and be stopped by slippage, exposing the crack tip to the corrosive solution. Subsequently, intergranular penetration, film formation, brittle cracking, and crack expansion will occur. This process is repeated. , eventually forming a stepped discontinuous fracture. In the non-film-forming solution, the stress causes the outcropping dislocations on the brass surface to preferentially dissolve, causing cracks to propagate along the path with the highest dislocation density and causing fracture. In brass with low zinc content, dislocations are mainly in the form of cells, and the grain boundaries are the areas of maximum dislocation density, so fractures occur along the crystalline form. In brass with high zinc content, dislocations are mainly in planar form, and stacking faults are the areas of maximum dislocation density, so transgranular fracture occurs. In addition, since zinc atoms segregate at dislocations under stress and increase the activity at dislocations, the crack growth rate will increase with the increase in zinc content.
Experimental research shows that among the atmospheres, industrial atmosphere is most likely to cause stress corrosion cracking of brass, and has the shortest fracture life; rural atmosphere is the second most likely, and marine atmosphere has the least impact. This different impact in the atmospheric environment is caused by the difference in SO2 content in the atmosphere (industrial atmosphere contains the most SO2, rural atmosphere contains less SO2, and marine atmosphere contains almost no SO2).
In short, the substances that cause stress corrosion cracking of brass are mainly ammonia and substances that can derive ammonia, or sulfides. Among them, the role of ammonia is recognized, while the role of sulfide is unclear. In addition, steam, oxygen, SO2, CO2, and CN- have an accelerating effect on stress corrosion.
(2) Stress
Tensile stress is a necessary condition for stress corrosion cracking in brass. The greater the tensile stress, the greater the stress corrosion cracking
The higher the sensitivity. Using low-temperature tempering to eliminate residual tensile stress can protect brass from stress corrosion cracking.
(3) Alloy composition and structure
The higher the zinc content in brass, the greater its susceptibility to stress corrosion cracking. As for how low the zinc content is, stress corrosion will not occur. This is related to the properties of the medium. For example, brass with a zinc content of less than 20% generally does not cause stress corrosion in the natural environment, while brass with low zinc content in ammonia water will also cause stress corrosion cracking.
The influence of other alloy elements on stress corrosion, Si can effectively prevent stress corrosion cracking of α brass. Si and Mn can improve the stress corrosion resistance of α+β and β brass. Under ammonia atmosphere conditions, Si, As, Ce, Mg and other elements improve the stress corrosion resistance of α brass. Under atmospheric conditions, Si, Ce, Mg and other elements improve stress corrosion performance. The results of industrial atmospheric exposure tests show that adding Ai.Ni and Sn to Cu-Zn alloys can reduce stress corrosion tendencies.
